One customary application for a solenoid is in the control of the fuel supply to a diesel engine, as in a diesel powered truck, automobile, or other vehicle. The solenoid plunger is connected to the fuel pump for the engine in an arrangement such that the solenoid plunger has its maximum extension when the solenoid is in its initial de-energized condition. For this operating condition, the fuel supply to the diesel engine is cut off and the engine cannot run. Whenever the engine is to be started, the solenoid is energized to retract its plunger, which conditions the fuel pump to supply fuel to the engine. Usually, a dual coil solenoid is employed, incorporating a pulling coil and a holding coil. The coils are energized to pull the solenoid plunger to a retracted position. When the movement of the plunger to its retraction position is completed, the pulling coil is de-energized but the holding coil remains energized to keep the plunger retracted. When it is desired to shut off the engine the holding coil is de-energized to allow a return spring for the solenoid to restore the plunger to its original intended position. Similar solenoid uses occur with stationary engines and in other applications.
In an arrangement of this kind, if there is an electrical failure in the solenoid or in the circuits connected to the solenoid, it may be impossible to energize the solenoid coils, particularly the pulling coil, to retract the plunger and allow operation of the engine. In these circumstances, with most solenoids it is essentially impossible to retract the solenoid plunger to allow operation of the engine long enough to get the truck or other vehicle to a location where repair can be conveniently effected. Usually, it is necessary to tow the vehicle to a repair facility. In industrial applications, a similar electrical failure may cause a costly interruption in a critical industrial process.